Fabrication of 2D Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays on Insulating Substrates With Through-Wafer Interconnects Using Sacrificial Release Process

Abstract

A critical component in a three-dimensional (3D) ultrasound imaging system is a two-dimensional (2D) transducer array. A 2D transducer array is also essential for the implementation of a compact form factor focused ultrasound system for therapeutic applications. Considering the difficulty associated with developing 2D transducer arrays using piezoelectric technology, capacitive micromachined ultrasonic transducer (CMUT) technology with the inherent advantages has emerged as a candidate to develop these devices. Previously, we demonstrated that 2D CMUT arrays can be fabricated with through-glass-via interconnects on borosilicate substrates using anodic bonding. In this paper, we present a fabrication process for implementing $16\times 16$ -element 2D CMUT arrays on an alkali-free glass substrate using the sacrificial release method. The vacuum-sealed $16\times 16$ -element 2D CMUT array is built on an SGW3 glass substrate with copper through-glass interconnects. The fabrication process developed for the 2D CMUT array is described in detail. Across the 256 elements of the 2D CMUT array, the mean resonant frequency is measured as 4.76 MHz with a standard deviation of 46.6 kHz. Also, the mean device capacitance across the array is measured as 1.17 pF with a standard deviation of 0.12 pF, and these results agree with the finite-element analysis. This study shows an alternative method to fabricate 2D CMUT arrays on glass substrates with metal interconnects, especially when the substrate is not suitable for anodic bonding. In addition to improved reliability and reduction in parasitic interconnect capacitance and resistance, this fabrication method benefits from the flexibility of developing 2D CMUT arrays on any type of insulating substrate, and still attain optimum uniformity in both yield and functionality of the fabricated devices. [2019-0246]